Protein content was measured spectrophotometrically according
to Bradford [25] and bovine serum albumin was used as a standard.
All measurements were performed in triplicate. The enzyme
activity was estimated according to modifications of the method
described by Kunitz [26] and Walter [27], as follows: 2% casein
(w/v) in 0.1 M phosphate buffer (pH 7.5) was used as a substrate.
Aliquots of 50 lL of the samples were added to a centrifuge tube
containing 0.625 mL of buffered solution of casein. The mixture
was maintained for 10 min in a water bath at 37 C. Subsequently,
1.25 mL of a solution of trichloroacetic acid (TCA) was added and
after 10 min at room temperature (25 C), the mixture was centrifuged
at 4000g for 20 min. The absorbance of the supernatant was
determined at 280 nm using a UV/visible spectrophotometer
(Beckman DU 640, USA).